The present invention relates to a print medium and a printing method for making recording material such as volatile dye sprayed by vaporization or ablation to adhere to the printing medium, more particularly to a printing paper for a dye vaporization type printer and a dye vaporization type printing method using such printing paper.
In recent years, needs of color printing as well as single color printing has been increasing for hard copies in recording images of video cameras, television sets, and computer graphics.
In response to those needs, hard copying methods such as dye thermal transfer method, wax type thermal transfer method, ink jet method, electrophotographic method, thermal developing silver halide method, etc. have been proposed. Among them, methods for outputting high quality images using a simple apparatus are roughly classified into dye dispersing thermal transfer method (dye sublimative thermal transfer method) and ink jet method.
Among those printing methods, according to the dye dispersing thermal transfer method, which is included in the dye sublimative thermal transfer method, an ink sheet coated with an ink layer comprising a high concentration thermal transfer material dispersed in proper binder resin is brought into close contact at a fixed pressure with a thermal transfer medium such as a piece of photographic paper coated with dye receptive resin that allows a transferred dye to be adhered, then heat corresponding to the image information is applied to the ink sheet by a thermal printing head positioned above the ink sheet. The dye is thus transferred onto the dye receptive layer from the ink sheet according to this heating.
The so-called thermal transfer method, in which the above operation is repeated, for example, for each of the image signals corresponding to the three subtractive primaries, yellow, magenta, and cyan, respectively, is a quick method for getting full-color images and widely noticed as an excellent technique for getting high quality images equal to silver halide color photography.
A thermal printing head (hereinafter referred to as a thermal head) is used for some types of thermal transfer printers. This method, however, has such disadvantages as to produce a large quantity of waste due to disposable ink ribbons (or ink sheets), and require expensive running cost, which prevents this method from spreading. This is also the same in the wax type thermal transfer method.
Furthermore, in full-color printing, a certain color ink once transferred on the printing paper is sometimes transferred back on an ink sheet of different color. This might result in impure printed images due to color mixture of the inks.
Although the thermal developing silver halide method can provide high quality images, the running cost and the apparatus cost are also expensive, since exclusive photographic printing paper and disposable ink ribbons (or ink sheets) are used.
On the other hand, the ink jet method, as disclosed in Unexamined Japanese Patent Applications No. 61-59911 and No. 5-217 carries out printing of images by spraying droplets of liquid dye from nozzles provided on the recording head so as to be adhered on the object printing paper using such methods as the electrostatic attracting force method, the continuous vibration generating method (piezo method), thermal method (bubble jet method), and others selected appropriately according to image information.
Consequently, the method enables image transfer onto normal paper almost without producing waste as is produced in using ink ribbons and the running cost is low. In recent years, the thermal method is becoming popular because the method can output color images easily.
With the ink jet method, however, it is basically difficult to provide density gradation in pixels. Thus, it is difficult to reproduce high quality images equivalent to silver halide photography in a short time as can be obtained by the dye dispersing thermal transfer method.
In other words, since in the related art ink jet printing method, one pixel is formed with one droplet of ink, providing the density gradation in pixels is basically so difficult that it cannot reproduce high quality images. Although the high resolution of the ink jet printing is utilized in the dither method for representing a pseudo gradation, but the same quality as that in the dye thermal transfer method cannot be obtained. Furthermore, the method lowers the image transfer speed significantly.
On the other hand, for the electrophotographic method, the running cost is low and the image transfer speed is high, but the apparatus cost is expensive.
As described above, there has not been any printing method that can satisfy the requirements for image quality, running cost, apparatus cost, image transfer speed, so far.
The inventors have disclosed a method and an apparatus for printing that are proposed to solve the above problems in Unexamined Japanese Patent Application, No. 7-89107. In this prior application heat generated by a heating source heats a dye via a heat medium for supporting the dye (e.g. a light-heat converter comprising carbon particles and binder or comprising a nickel-cobalt alloy thin film and the dye is vaporized or sublimated to be transferred onto the printing paper held with a gap of 1 to 100 .mu.m between the dye and the printing paper.
In other words, in the method of printing according to the invention of the prior application, a porous structure is formed at the dye heating part of the printer, and the porous structure provides an increased surface area of the heating part (transfer part) that can keep supplying the liquid dye to the heating part and hold the dye there due to the capillarity. And, an amount of heat corresponding to the information for printing is added selectively by a heating means in this state to vaporize part of the liquid dye and transfer an amount of dye onto the printing paper in a form of steam or fine droplets to make a copy according to the information for printing corresponding to the electrical images created by a color video camera.
Consequently, when compared with the well-known ink jet method, this method can form a number of fine liquid droplets and control the number of those droplets freely according to the heating energy applied to the dye liquid heating part corresponding to the information for printing, so that multiple-value density gradation is enabled to obtain printed images (e.g. full-color images) of the quality equivalent to or higher than that of images of the silver halide printing method.
Furthermore, since this printing method adopts vaporization or sublimation of dye, it does not need heating of the dye receptive layer of the print medium as the thermal transfer method in the related art. It is also not needed to press the ink sheet against the print medium strongly nor to use any ink sheet (or ink ribbon). This is also very advantageous to reduce the size and weight of the printer, as well as to reduce the quantity of waste. Furthermore, because the dye layer of the vaporization part is kept separated from printing paper, neither thermal sticking nor color mixture caused by transfer back as mentioned above occurs between them. Furthermore, the printing is possible even when the mutual solubility between dye and dye receptive layer resin is small. Thus, the design and selection of dye and dye receptive layer resin can be carried out with much freedom.
Any dye can be used for this printing method if it has a proper vaporization or ablation speed, exhibits fluidity at 200.degree. C. or under when used independently or mixed with others, and has a necessary and sufficient heat resistance. Specifically, the dye may be a disperse one, an oil-soluble one, a base one, an acid one, and the like. Even if a dye has a melting point above the room temperature, when it is mixed with another dye or a volatile low molecular compound, the mixture provides a lowered melting point.
Any photographic papers can be used for this printing method if it has a proper compatibility with the transferred dye, easily accepts the transferred dye to enhance development of the natural color of the dye, and works to fix the dye. For example, for a disperse dye, a piece of paper whose surface is coated with polyester resin, vinyl chloride resin, acetate resin, or the like that has good compatibility with the disperse dye would be preferable. Fixing of a dye transferred onto photographic paper is possible with a method for heating transferred images to permeate the transferred dye on the surface into the dye receptive layer.
Printing method by this dye vaporization (or ablation) method has the advantage of providing a printer with reduced size, easiness in maintenance, quickness in printing, and performance for producing images with enhanced quality level and enhanced gradation.
However, there has been found no paper suitable for dye vaporization (or ablation) method printers having the above mentioned excellent advantages so far. In other words, if conventional printing paper is used for such a printer, the following problems will arise.
(1) Normal paper (copying paper)
Image transfer is possible for this paper, but much ink runs on the paper, resulting in unclear print images.
(2) Ink jet printing paper
A surface layer with hydrophilic resin as a binder is formed on the paper material, but adhesion of the hydrophobic dye used for the dye vaporization method is poor for this surface layer, resulting in low printed density of printing and poor image retention properties.
(3) Photographic paper for the dye sublimative thermal transfer (thermal head) method
Since hydrophobic resin is coated on synthetic paper such as plastic film, etc., the adhesive property of the hydrophobic dye used for the dye vaporization method is satisfactory, and printed images are clear with the high printed density. However, the ink absorption time becomes as long as a few tens of minutes. Furthermore, when images are touched before the ink is absorbed, the images are damaged. Furthermore, since photographic paper is pressed against the thermal head, the paper must have an enough mechanical strength. Consequently, the paper thickness must be increased. The paper is also needed to be heat resistant. The cost of such photographic paper becomes high for those reasons.
Under such circumstances, it is an object of the present invention to provide a print medium such as printing paper, etc. that can make the printing apparatus such as the dye vaporization (or ablation) type printer mentioned above exhibit its excellent characteristics and a printing method using such a print medium.